1. Field of the Invention
The present invention relates to a novel method for preparing methylene-1,3-dioxolanes. Methylene-1,3-dioxolanes are important intermediates for preparing pyrazoles and anthranilic acid amides, which may be used as insecticides.
2. Description of Related Art
As previously described in the literature, 2-methylenedioxolanes may be prepared from 2-chloromethylenedioxolanes and KOH. It is reported in Journal of Polymer Science 1964, vol. 2 p.3471 and Biochemical preparation, 1960 v. 7, 45, that the reaction of 4-chloromethylene-1,3-dioxolanes with solid KOH affords only approx. 52% yield of the product (4-methylene-1,3-dioxolanes) and approx. 60% yield of 2,2-dimethyl-4-methylene-1,3-dioxolane. In contrast, Gevorkyan et al, Khimiya Geterocycl. Soed. N 1, 1991, pp. 33-36 report that KOH, when used as a solid or under phase transfer conditions, is not suitable for the dehydrochlorination of 2-chloromethyldioxolanes. For example, Gevorkyan et al. describe an isomerisation in which a shift in the double bond in the ring occurs. The elimination of HCl using anhydrous sodium diethylene glycolate (prepared from Na and diethylene glycol) was also proposed by Gevorkyan et al, Khimiya Geterocycl. Soed.N 12, 1983, 1607-1613, in which yields of approx. 70-80% can be achieved. The use of metallic sodium in an industurial setting is unfavourable for safety reasons. During the elimination of HCl some of the corresponding chloromethylenedioxolanes react with substitution of the chlorine atom by ethylene glycol anion, producing a high-boiling compound. It is reported in J. Org. Chem, 1987,52, 2625-27 that 2,2-dimethyl-4-methylene-1,3-dioxolane has only limited stability at −10° C. This indicates that the stability of the product is dependent on the production method.
In the methods described in the literature further disadvantages have been noted. The use of excess solid NaOH or KOH without solvent renders the reaction mixture impossible to stir, particularly towards the end of the reaction, after the product has been partly distilled off. It has additionally been observed that isomerisation occurs in the case of 2-methylene-4,4-dioxolane, particularly when the procedure is scaled up, where up to 5% of the corresponding isomer may form. Furthermore, the product partially decomposes under the reaction conditions with formation of acetone. This significantly influences the purity of the product, particularly on scale-up, which contains acetone residues. Up to 4% of acetone has been observed.